For sound effects, you’ll need to rip them from FE8 using Sappy, import the sample into FE7, create a MIDI that plays a single note with the same duration as the sound, edit the voice table so that it uses the imported sample, set the sound ‘priority’ to 6, and finally use SXXXX in your animation script to play the sound.
See if this helps: Playing custom sounds in battle animations
Normally, sound effects will take priority over background music, but for some reason imported sounds have lower priority than background music. You should be fine if you’re using the 12 sounds patch, but it’s something to be careful of.